Contact-drive vibratory fork



y 19300 I A. M. CURTIS 1,757,704

CONTACT DRIVE VIBRATORY FORK Filed 001;. 29, 1927 AECDEFG/fl- MENTOR Ausmw M. (Tl/H775 Patented May 6, 1930 UNITED 'rl'TATES [PAT-ENT- orrlca mm: M. cun'rrs, or EAST omen, NEW .neznsnv, ASSIGNOR 'ro BEL rnnnrnonnl LABORATORIES, mconronA'rEn, or NEW YORK, N. Y., A, CORPORATION'OF NEW YORK A CONTACT-DRIVE vnamironv' FORK Applioation filed October 29, 1927. serial No. 229,772.

This invention relatesto electrically driven tuning forks and its object is'a fork'that combines the simplicity and cheapness of the conventional contact drive fork with a degree of 8 accuracy and reliability in operation hereto- 7 an electromagnet excited from a source of direct current received through a fixed contactand a contact on the fork prong, in the same manner as a simple buzzer is actuated. .That type of fork has the serious disadvantages that the driving force is applied to thefork tine as a long impulse when the tine is near the end of its swing and that the length. of

the impulse depends on the amplitude of vibration. Thus if from any cause, such "as a variation in the voltage of the power supply,-

the amplitude of vibration increases,-the

longer contact closure tends to still further increase thejamp'litude. Since the frequency voltage produce excessive variations in the frequency. That type'of drive furthermore, requires a relatively heavy exciting current, which must be broken by the fork contacts. This sometimes causes sticking and burning of the contacts which not only 'necessitates g contact 5. A driving. electromagiiet (it is mounted in inductiverelation to prong 2 and frequent attention but also tends to make the fork erratic in operation. It has been realized before that a driving impulse applied to the fork prong as it passes through its normal position of rest, would be much more effective and would require a smaller effective oper-" ating current. Some systems have been 'de-.

veloped which utilize "this principle but they have required rather complicated mecha nisms which more or less defeated the attainment of a reliable, accurate fork.

The secondtype mentionedabove is the vacuum tube regenerative fork in which the nected in the driving circuit that it is charged of vibration of a tuning fork is a function of j the'amplitude, slight variations in the supply motion of the fork changesthe flux in the core of an electromagnet placedin inductive relation thereto and. generates a weak alternating current in the winding. This weak current is amplified in a vacuum tube amplifier and applied to the fork-driving-electromagnet. With such an arrangement all ourrentbreaking contacts are eliminated and vacuum tube regenerative forks are noted for their accuracy and reliability. However, they are more expensive and more complicated than ordinary contact drive forks.

This invention comprises a contact drive fork in which the driving impulse is applied to the fork tine when it is\passing through its. normal position of rest, and in which the contacts aria not required to break a current durving normal operation. The above effects are obtained by utilizing a condenser so -cononce durin each cycle, through-aback contact of'the fork and subsequently discharged through a front contact of the fork and the driving electromagnet as the fork tine passes hrough its mid-point.

The invention will nowbe explained in connection with thedrawing, in which;

. Fig. 1 shows a preferredembodiment'of a circuit embodying the invention; v i

. Fig. 2 containscurves illustrating the operation of the circuit of Fig. 1; and a Figs. 3,-3A, 4 and 5 show various types, of contacts that may be used on the tuningfork of Fig. 1-. r

, Referring to Fig. 1 a tuning fork -1 has a tine 2 carrying a contact. 3; arranged togenage a fixed back contact 4 or a fixed front has its windin connected between contact 5 00 and one pole 0 battery 16. A transformer-14 .having a primary winding 13' and a secondary winding 15 is used to supply a load circuit with alternating current of thefrequency of vibration of the fork. A large condenser ll-0f several microfarads capacity is connected in series with the transformer primary winding 13 and is an important feature. of the system. Resistance 7 and con- 'denser 8 shunted across driving magnet 6 are '10) effectively remove from the circuit including condenser 11, the large inductance of the transformer primary winding. Resistances 7, 9, 10 and 12 and condenser 8 may be disre garded in following the operation of the system. z

The'operation of the system shown in Fi 1 will now be explained in connection wit the curves of Fig. 2. Curve 24 represents the vibration of the fork and curve 23 the current through the contact 3, both plotted against time. Assume that the for ,isin vibration and that at time A (see Fig. .2) the prong 2 is moving away from driving magnet 6 and that moving contact 3 hasjust 'broken contact with fixed contact 5. 'The fork prong2 continues its swing upward and at time B as reached its uppermost position, in which contact. 3 makes on contact4 and closes a circuit fromthe upper plate of condenser 11, through the tuning fork, contact 3, contact 4, battery 16, and the primary winding of transformer 14' to the lower plate .of condenser-11, and charges condenser 11 to Q the potential of'batter'y 16. Curve 23 represents' the direction and time duration of current flowing in the condenser'circuit, .charg ing impulses being represented as positlve values and discharges. as negative. Tine 2 having'co'mpleted its upward swing returns to its mid-point as represented at C in Fig.2

and causes contact 3 to closeon contact 5. This closes a circuit from condenser 11 through the tuning fork and contact 3, contact 5, driving magn'etfi, resistance 9, and primary w1nding13 ofthe output transformer 14 to the other plate of condenser 11. Condenser 11. therefore discharges through the circuit 'ust described, as disclosed in the curve 23 of ig. 2 and energizes driving magnet 6 which thus momentarily-attracts the tine 2. The fork' tine 2 continues its travel and at time D has reached its lowermost position and at time E has again returned to the midpoint and causes contacts 3 and 5 to open. Since the discharge 'of condenser 11 was of only momentary durationnocurreint is flowing across contacts 3 and 5 at time E when the break, and no destructive spark- 7 ing can ta e lace.

Theoretica y there is practically no current flowing through the contacts at the time theyare broken and this condition would be attained in practice providing chattering could be entirely eliminated. Actually, it is very difficult to obtain a moving contact such as shown in 3 of- Fig. 1 that will not rebound or bounce slightly from impact with the fixed contacts. Since this rebound may occur before the condenser 11 has completely charged 2 or discharged it may result'in some slight sparking at the contacts. To reduce chattering to the minimum special contacts have been devised and are illustrated in Figs. 3, 3A, 4 and.5. A

Of the three types of contacts shown, that in Fig. 5 is to be preferred. It consists of a flexible ribbon 3 woven of bronze wire which is rigidly attached to the fork prong 2 and carries a welded contact point 22 at its free end. The woven bronze ribbon 3 is flexible and elastic, but is effectively damped by the internal friction between the individual strands, of which it is composed, so that it has little or no tendency to vibrate at a natural frequency of its own. I In the'arrangement of arm- 3 is pivoted ,to swing freely between the fixed contacts 4 and 5. It is not attached to the fork tine, but is coupled thereto by magnetic attraction, a small permanent ma et 18 being mounted at the end of the fork, as

L Fig. 3 thecontact v shown to better advantage'in Fig. 3A. Since contact arm 3, or at least the end thereof, is

The arrangement of Fig. 4 is similar to,

that of Figs. 3 and 3A, in that the contactarm 3 is coupled to the fork by magnetic at-- traction,but differs from it in the method of reducing chatter. In'Fig. 4 the element 3.

which is of magnetic material is mounted to a vibrate between poles 20 and 21 of a permanent magnet 19. Since the attraction exerted on the element 3 by either pole of the permanent magnet 19 varies inversely as the distance between them, it will tend to hold contact arm 3 firmly a ainst whichever fixed contact 4 or 5 it may e touching, and thus pre-,

vent chattering. The magnetic attraction between magnet 18 and element 3 is the controlling force. The magnet poles 20 and 21' serve merely treme positions.

. Tests have revealed that the average value of currentrequired to operate a fork of the type disclosed is about one-tenth of that re-- quired for the ordinary type of contact drive fork. Thus, sparking and arcing at the contacts would be greatly reduced, even if-current were flowing at the time of contact break,

but since with anefiicient anti-chatter conand burning of the contacts isflargely elim-- to hold the element 3 in its exinated. Furthermore, the driving impulse is dependent only on the capacity of the condenser 11. and the electromotive force of the battery 16, whereas, as pointed out before,

in the ordina fork the time during which current is app 'ed depends upon the amplitude of vibration as well as the electromotive force of the battery. This results in a fork that vibrates at a relatively constant frequency and yet retains the simplicity and ifzhgk low cost of the ordinary contact drive What is claimed is:

1. The method of driving a vibrating eleent which comprises storing electrical enerand subsequently releasing it to apply a iving impulse to said element as it is passing through its normal position of rest.

2. A vibratory element and electrically operated driving means therefor, the operation of which is controlled by contacts controlled by the motion of the vibratory element, and

means for preventing a flow of current through said contacts'at the time they are broken.

3. In combination with a vibratory element, electrical driving means therefor, a source of electrical energy, a condenser, electrical switching means adapted to connect said source to said condenser when the vibratory element is near the end of its swing in one direction, and to connect said condenser to said driving means when the vibratory element is passing through its normal position of rest.

4. In combination with a tuning fork, a driving electroma et in inductive relation thereto, a source 0 current, a condenser, and contacts controlled by the motion of the fork whereby said source is connected to the condenser during one phase of the forks motion and the condenser is subsequentl connected to the driving electromagnet urin-g another phase of the forks motion. v

5,. In combination with a tuning fork, a driving electromagnet in inductive relation thereto, a source of direct current, a con-' denser, a fixed contact connected to one ter- 1 minal of said source, a second fixed contact connected through said driving electromagnet to the other terminal of said source, a contact movable between the fixed contacts under the control of the fork and connected through the condenser to said other terminal of said source.

6. In combination, a vibratory element, means for vibrating said element comprising a contact attached to said element by a flexible supporting arm consisting of a woven wire ri bon.

In witness whereof, I hereunto subscribe my name this 27th day of October, A. D.,

AUSTEN M. CURTIS. 

